Colour constancy failures expected in colourful environments

David H Foster,Adam Reeves

doi:10.1098/rspb.2021.2483

Abstract

Colour constancy refers to the constant perceived or apparent colour of a surface despite changes in illumination spectrum. Laboratory measurements have often found it imperfect. The aim here was to estimate the frequency of constancy failures in natural outdoor environments and relate them to colorimetric surface properties. A computational analysis was performed with 50 hyperspectral reflectance images of outdoor scenes undergoing simulated daylight changes. For a chromatically adapted observer, estimated colour appearance changed noticeably for at least 5% of the surface area in 60% of scenes, and at least 10% of the surface area in 44% of scenes. Somewhat higher frequencies were found for estimated changes in perceived colour relations represented by spatial ratios of cone-photoreceptor excitations. These estimated changes correlated with surface chroma and saturation. Outdoors, the colour constancy of some individual surfaces seems likely to fail, particularly if those surfaces are colourful.

Highlights

For an object or surface to be recognizable by its colour, it should appear or be perceived as the same independent of the accident of illumination, whether, for example, from a blue sky or a yellow-orange sun
The results suggest that outdoor environments may well contain individual colourful surfaces or parts of surfaces that fail to be colour constant
Scene data were taken from a set of 50 hyperspectral images of outdoor scenes drawn from the main land-cover classes [30,31], either predominantly vegetated, containing woodland, shrubland, herbaceous vegetation and cultivated land, or predominantly non-vegetated, containing barren land, urban development, as well as farm outbuildings, and painted or treated surfaces

Summary

Introduction

For an object or surface to be recognizable by its colour, it should appear or be perceived as the same independent of the accident of illumination, whether, for example, from a blue sky or a yellow-orange sun. The hue difference between the purple and orange flowers under the 6500 K illuminant appears smaller under the 4000 K illuminant, an effect which is largely independent of adaptation. This kind of change has been taken as a failure of relational colour constancy [15,19]. Most outdoor scenes do not consist of orange and purple flowers [20,21,22,23,24,25], nor do their gamuts span the spaces of Munsell or model reflectance spectra mentioned earlier. The results suggest that outdoor environments may well contain individual colourful surfaces or parts of surfaces that fail to be colour constant

Methods

Findings

Discussion